External beam radiotherapy for breast cancer is typically delivered by opposing tangential fields which provide a uniform dose to the entire affected breast. The treatment is given over a number of sessions, and is often followed by additional boost sessions. The boost sessions are typically delivered with an electron beam, which is designed to treat the primary lumpectomy site only.
Unlike photons, whose intensity decreases in an approximately exponential fashion within the patient, electrons deposit most of their dose within a fixed, finite range which depends on the energy of the beam. Thus, a single electron beam can be used to treat superficial lesions while sparing underlying healthy tissues. Electron treatments are delivered with electron cones of various sizes and shapes that are typically attached to the collimator of a linear accelerator, and which shape the electron beam very close to the patient surface. The shapes can be standard geometric shapes, such as circles or squares of various sizes, or an arbitrary shape can be custom-made for a given patient. In some instances, a lead sheet having an opening that defines the aperture of the beam is placed directly on the patient's skin.
Electron treatments are usually planned with a fixed source-to-skin distance (SSD). Breast boosts (a radiotherapy treatment in which a “boost” of 10-16 Gy of radiotherapy is given in addition to the normal radiotherapy treatment after surgery) typically use an SSD of 100 cm, as this is the same distance from the beam source to the isocenter of most linear accelerators (“linacs”). As a result, the linac isocenter, and hence the intersection of any wall lasers being used to align the patient with the linac, lies on the patient skin surface. This is in contrast to many photon treatments, which are planned such that the isocenter is near the center of the treatment volume.
For a breast boost, the electron field ideally should cover the tumor bed and the surgical path leading from the tumor bed to the surgical scar, plus a 1-2 cm margin. In addition, it is preferable to avoid the areola. Unfortunately, the location of the scar, which is used often as a proxy for the lumpectomy site to aim the electron beam, is often a poor indicator of the actual location of the underlying tumor bed. Ultrasound has been used for planning purposes to obtain the size and shape of the tumor bed, or seroma, which is the fluid-filled region of the lumpectomy site. Surgical clips, placed during surgery around the lumpectomy site, are radio-opaque and have also been used as a proxy for the lumpectomy site for planning purposes. What is needed, however, is a methodology to incorporate internal information, e.g., the position and extent of seroma as observed in ultrasound images, into the conventional clinical setups (simulation and/or treatment) for electron breast boost treatments.